A variable displacement reciprocating compressor used in an automotive air conditioning system, for example, has a housing, and inside the housing, a discharge chamber, a suction chamber, a crank chamber and cylinder bores are defined. On a drive shaft extending inside the crank chamber, a swashplate is mounted to be variable in inclination, and a conversion mechanism including the swashplate converts rotation of the drive shaft into reciprocating motion of pistons fitted within the respective cylinder bores. By the reciprocating motion, each piston performs a discharge process of drawing a working fluid from the suction chamber into its own cylinder bore, compressing the drawn-in working fluid and discharging the compressed working fluid to the discharge chamber.
The length of the stroke of the piston, therefore, the displacement of the compressor can be varied by varying pressure in the crank chamber (control pressure). In order to control the displacement, a displacement control valve is arranged in a gas supply passage connecting the discharge chamber and the crank chamber, and a restriction is provided between the crank chamber and the suction chamber.
In the displacement control valve, as disclosed in document 1 (Japanese Patent Application KOKAI Publication 2002-285973), for example, by controlling activation of a solenoid depending on the operating state of an engine, etc., a valve element is moved to open or close the valve. By this, supply of a working fluid from a discharge chamber to a crank chamber is controlled, so that the displacement of the compressor is varied.
For the displacement control valve shown in FIG. 2 of document 1, the relationship among forces acting on the valve element 25 is represented by equation (1) below. Equation (1) can be rearranged into equation (2) giving an acting pressure difference ΔP (difference between discharge pressure Pd and suction pressure Ps). Here, Sv is the area of that surface of the valve element which receives the discharge pressure, Pd is the discharge pressure, Ps is the suction pressure, f1 is a force exerted by a compression coil spring 28, f2 is a force exerted by a compression coil spring 27, and F(I) is an electromagnetic force generated by a solenoid supplied with a control current I. It is arranged that f1 and f2 satisfy a relationship f1>f2.
                                          Sv            ·                          (                              Pd                -                Ps                            )                                +                      f            ⁢                                                  ⁢            1                    -                      f            ⁢                                                  ⁢            2                    -                      F            ⁡                          (              I              )                                      =        0                            (        1        )                                          Δ          ⁢                                          ⁢          P                =                              Pd            -            Ps                    =                                                    1                Sv                            ·                              F                ⁡                                  (                  I                  )                                                      -                                                            f                  ⁢                                                                          ⁢                  1                                -                                  f                  ⁢                                                                          ⁢                  2                                            Sv                                                          (        2        )            
Provided that the solenoid is designed to generate the electromagnetic force represented by F(I)=A·I (A is a coefficient), equation (2) can be rewritten into equation (3) below. FIG. 6 shows the graph of equation (3).
                              Δ          ⁢                                          ⁢          P                =                              Pd            -            Ps                    =                                                    A                Sv                            ·              I                        -                                                            f                  ⁢                                                                          ⁢                  1                                -                                  f                  ⁢                                                                          ⁢                  2                                            Sv                                                          (        3        )            
FIG. 6 shows that the acting pressure difference ΔP, namely Pd−Ps is in proportion to control current, and that a maximum acting pressure difference ΔPmax requires a maximum value Imax of control current. The acting pressure difference ΔP thus varies from 0 to ΔPmax as the control current is regulated in the range of 0 to Imax.
Provided that Imin represents a control current value at which Pd−Ps becomes 0, Imin==(f1−f2)/A is derived from equation (3). Since f1>f2, the valve element stays in an open position for values 0 to Imin of control current I. In order for the displacement control valve to function, supply of control current I of Imin or greater is required. In sum, the arrangement that the force resulting from the two compression coil springs and the electromagnetic force generated by the solenoid act in opposite directions does not allow effective use of the electromagnetic force generated by the solenoid, from the control current value 0.
Further, Imin being not 0 leads to a great gradient of the acting pressure difference ΔP varying depending on the current, which means a slight variation in control current I results in a significant variation in Pd−Ps.
Further, the great gradient of the acting pressure difference varying depending on the current means that the coefficient A/Sv of the current I in equation (3) needs to be great in order to achieve the maximum acting pressure difference ΔPmax. This requires that Sv be small. Sv is the area of the surface which receives the discharge pressure, and at the same time the area of the surface which receives the suction pressure. Smaller area Sv results in lower sensitivity of the valve element responding to variations in discharge pressure or suction pressure, which may impair the stability of displacement control.